1. Field of the Invention
This invention relates to a driving force control system for a four-wheel drive vehicle, which controls the engagement forces of clutches to thereby control the drive forces distributed to auxiliary drive wheels.
2. Description of the Prior Art
A driving force control system of this kind has been proposed by the present assignee in Japanese Laid-Open Patent Publication (Kokai) No. 10-194005, for a four-wheel drive vehicle including left and right front wheels as main drive wheels, and left and right rear wheels as auxiliary drive wheels. The engine of the vehicle is connected to the left and right front wheels via an automatic transmission, a front differential, and left and right front drive shafts. Further, the front differential is connected to the left and right rear wheels via a transfer, a propeller shaft, a rear differential, and left and right drive shafts.
The driving force control system includes left and right electromagnetic clutches provided for the rear differential, an ECU for control of these electromagnetic clutches, and a lock switch arranged on a dash board. The ECU controls the supply and cut-off of electric current to the solenoid coils of the respective clutches, whereby the clutches connect and disconnect the associated rear drive shafts to and from the propeller shaft. When these clutches are disengaged, the driving force of the engine is transmitted to the front wheels alone to thereby bring the vehicle into the front-wheel drive mode, whereas when the same are engaged, the driving force of the engine is also distributed to the rear wheels to bring the vehicle into the four-wheel drive mode. The engagement forces of the electromagnetic clutches are continuously changed according to the amounts of current supplied thereto, and the ECU controls the amounts of supplied current to vary the respective engagement forces of the left and right electromagnetic clutches whereby the diving forces distributed to the left and right rear drive wheels can be controlled independently of each other, as desired.
Further, when the driver operates the lock switch, the ECU executes a lock mode in which the amounts of current supplied to the clutches are made maximum to generate the maximum clutch engagement forces so as to maximize the driving forces distributed to the rear wheels. This makes it possible, for instance, to easily extricate the vehicle from a stuck condition on a snowy road.
However, the conventional driving force control system is configured such that the lock mode is unconditionally executed when the lock switch is operated. Therefore, if the driver operates the lock switch without the knowledge of the function of the lock switch or by mistake, the lock mode is unnecessarily executed. As a result, during this lock mode, the maximum amount of current continues to be supplied to the electromagnetic clutches to waste the electric power, and further fuel economy is degraded due to traveling of the vehicle in the four-wheel drive mode, which is fuel-consuming and, in this case, unnecessary. This can adversely affect the drivability as well, depending on the conditions of the vehicle. Further, the clutches are operated at their maximum engagement forces frequently and over extended time periods, so that the durability of the clutches is also degraded, which will offer an obstacle to the reduced size of electromagnetic clutches.
Further, in the conventional driving force control system, the lock mode is executed in a state where the clutch engagement forces are fixedly held at a maximum. On the other hand, after the vehicle gets unstuck from a stuck condition, the vehicle sometimes enters a traffic-congested traveling condition in which the running of the vehicle is controlled by engine brake or foot brake, with the accelerator pedal being scarcely stepped on by the user, and its lock switch being kept ON. In such a case, the engine output torque assumes a negative or small value, and hence it is almost unnecessary to distribute the driving force delivered from the front wheels to the rear wheels. Despite the fact, the vehicle continues to be in the lock mode in which the maximum clutch engagement forces are generated, and hence a large amount of electric power-is wastefully consumed due to supply of the maximum amounts of current to the electromagnetic clutches.